Finally, we recall that in large parts of the regions of the CMSSM, NUHM1 and NUHM2
parameter spaces favoured at the 68% CL the relic \(\tilde{\chi }^0_{1}\)  density is brought into the
range allowed by Planck [1]} and other data via coannihilation with the stau and other sleptons
[2]}, [3]}.
In an SU(5) GUT, the left- and right-handed sleptons are in different representations,
\(\mathbf {\bar{5}}\)  and \(\mathbf {10}\) , respectively. Hence they have different masses, in general,
providing more flexibility in the realization of coannihilation.
Specifically, as mentioned above, the
freedom to have \(m_5 \ne m_{10}\)  allows the possibility that the right-handed up- and
charm-flavour squarks, \({\tilde{u}_R}\)  and \({\tilde{c}_R}\) , are much lighter than the other squarks,
opening up the novel possibility of \({\tilde{u}_R}/{\tilde{c}_R} - \tilde{\chi }^0_{1}\) 
coannihilation, as we discuss below.
